1. Field of the Invention
The present invention relates to a semiconductor device having an overcurrent protective function of a semiconductor switching element, and particularly relates to prevention of an erroneous operation of an overcurrent protection circuit of the semiconductor switching element, with a mirror period present immediately after turning-on.
2. Description of the Background Art
As a semiconductor device provided with a semiconductor switching element (hereinafter simply referred to as “switching element”), such as an inverter, there is a semiconductor device having an overcurrent protection circuit which performs a protection operation, such as cutting off of the switching element, to protect the switching element when a main current flowing through the switching element exceeds a certain level.
Methods for detecting the main current flowing through the switching element include a direct method for detection by connecting a shunt resistor to a main electrode of the switching element, or the like, and an indirect method for detection by dividing part of the main current into a current detection element (cell) connected in parallel with the switching element and detecting the divided current (sense current). A system using the current detection element (current sense element) has advantages as compared with a system using the shunt resistor, in that power loss due to the shunt resistor does not occur and a system size is reducible.
Generally, in a semiconductor device adopted with the system using the current sense element, dimensions and electric characteristics of the current sense element are set such that a ratio (current division ratio) of the sense current to the main current of the switching element is about 1/1000 to 1/100000. The sense current is converted to a voltage (sense voltage) by use of a resistor or the like, and inputted into the overcurrent protection circuit. In the case of the sense voltage exceeding a predetermined threshold, the overcurrent protection circuit determines that an overcurrent has flowed through the switching element, and performs a protection operation such as cutting off of the switching element to prevent damage on the switching element. A value of the main current of the switching element, with which the overcurrent protection circuit starts the protection operation, is referred to as a short circuit protection trip level (hereinafter referred to as “SC trip level”.
Unexamined Japanese Patent Application No. 2008-206348 discloses a semiconductor device capable of adjusting a SC trip level in accordance with an element temperature.
Immediately after turning-on of the switching element such as an IGBT or a MOSFET, there is a period referred to as a “mirror period” in which a gate-emitter voltage is held constant in order to charge a gate-collector capacity while the gate current charges a gate-emitter capacity.
Although a current division ratio of the sense current to the main current flowing through the switching element is desirably constant, the current division ratio differs between the switching element being in a mirror-period state and the switching element being in a steady state, and typically, the sense current flowing through the current sense element is larger (i.e., the current division ratio is larger) during the mirror period. Hence the sense voltage to be inputted into the overcurrent protection circuit also increases during the mirror period.
This phenomenon is problematic especially when a free wheel diode (FWD) for keeping a free wheel current (backflow current) flowing through an inductive load at the time of turning-off of the switching element is connected to the switching element. That is, when the switching element is turned on in a state where the free wheel current is flowing between the inductive load and the free wheel diode, a current with almost the same magnitude as that of the free wheel current flows during the mirror period immediately after the turning-on. Since the sense current flowing through the current sense element is larger during the mirror period than in the steady state, erroneous overcurrent detection occurs due to the sense voltage exceeding a threshold even when the current flowing through the switching element is not higher than the SC trip level, thereby causing the overcurrent protection circuit to perform an erroneous operation.
As a method for preventing this erroneous operation, there have hitherto been used a method for providing the overcurrent protection circuit with a low-pass filter of a time constant that corresponds to the length of the mirror period, a method (Leading Edge Blanking) for masking the sense voltage to be inputted into the overcurrent protection circuit during the mirror period, or the like.
However, with these methods for preventing the erroneous operation, the overcurrent protection circuit does not operate or a delay occurs in start of the operation during the mirror period immediately after turning-on of the switching element. This brings about a problem of delays in overcurrent detection and protection operation in such an abnormal situation as a short circuit in upper and lower arms or a short circuit in an H bridge of a three-phase inverter in a load where an overcurrent flows immediately after turning-on of the switching element.